Reference electrodes serve to deliver a constant reference potential for measurements in potentiometric measuring cells having one or a plurality of measuring electrodes. They are frequently used in many applications as rod shaped reference electrodes or, combined with a measuring electrode to form what is called a single rod, measuring chain. Measuring electrodes, together with which the reference electrodes are applied, include, for example, pH electrodes or ion selective electrodes for determining cations such as sodium, potassium, calcium or anions such as chloride, fluoride, nitrate and carbonate. For example, such electrode combinations serve to determine corresponding ion concentrations in aqueous solutions or media containing water, as well as in natural water, swimming pools, waste water, or product streams.
It is known that the part of the reference electrode, which is brought into contact with a sample (also referred to as the measured medium in the following) when performing the measuring, must assure an electrolytic contact of the reference electrolyte in the reference electrode with the sample. This contact location, where the liquid contact between the reference electrolyte of the reference electrode and the measured medium occurs, is referred to as the diaphragm. Frequently the diaphragm is embodied as a plug made from a networked hydrogel, as a porous ceramic or plastic dowel, as a gap or as ground glass.
As is known, the ion concentration in the measured medium is determined with a measuring cell constructed of a measuring electrode, namely an ion selective electrode or a pH electrode, and a reference electrode, based on a change of the potential difference between the measuring electrode and the reference electrode. The potential of the measuring electrode is dependent on the concentration of the ion type to be determined in the measured medium and, in an ideal case, is not influenced by the presence of disturbing ions, while the potential of the reference electrode is not influenced by the concentration of the type of ion to be determined or the disturbing ions. Accordingly, the potential difference between the measuring electrode and the reference electrode changes in this ideal case exclusively due to the change of the potential of the measuring electrode as a result of changed concentrations of the ions to be determined, while the potential of the reference electrode must remain unchanged, so that the concentration of the ion to be determined in the sample solution can be directly read off based on the potential difference following a corresponding calibrating.
Accordingly, a change of the potential of the reference electrode leads to a corruption of the measurement results. The operative region for such a change of the potential of the reference electrode is the region of the diaphragm, in which the reference electrolyte contained in the reference electrode is in direct or indirect liquid contact with the measured medium.
Loss of reference electrolyte from the reference electrode in the region of the diaphragm leads to a decrease in the concentration of the reference electrolyte, as long as it is not cancelled by an external electrolyte supply or a supply of undissolved salt in the reference electrolyte. For example, since the voltage of a silver/silver chloride reference electrode (also known as a Ag/AgCl reference electrode) filled with potassium chloride solution (i.e. a reference electrode, which most often contains a 3 molar potassium chloride solution as a reference electrolyte and has a sensor system in fixed contact with silver chloride, e.g. a silver chloride coated silver electrode) depends approximately on the logarithm of the concentration of the potassium chloride; a decrease in the concentration of the potassium chloride is associated with an increase of the electrode potential of the reference electrode, which in turn is noticeable as drift of the measuring chain voltage or the measured value. In measurements of the concentration of univalent ions by means of ion selective electrodes, a voltage measurement error of only 1 mV already corresponds to a relative concentration measurement error of 4%. In the case of online measurement technology, which is used, as a rule, in process measurement technology, the reference electrode is continually immersed in the measured medium. In this case, the decline in the concentration of the reference electrolyte can limit the lifetime or the service life of the reference electrode in the measured medium or require frequent recalibration or readjustment of the measuring chain in which the reference electrode is a component.
Due to the solubility of silver chloride in the relatively highly concentrated potassium chloride solution, the reference electrolyte of a silver/silver chloride reference electrode has, in general, 0.3 to 1 g/l dissolved silver chloride. If this reference electrolyte comes in contact with a measured medium which contains proteins, sulfides, iodides or other components, which react with silver to form a difficulty soluble compound, then these difficulty soluble silver compounds precipitate and clog the pores of the diaphragm. Also, in given cases, suspended materials present in the measured medium or other macroscopic fouling of the measured medium can contaminate the diaphragm.
Also, strongly oxidizing or reducing materials, which reach the housing interior of the reference electrode through the electrolytic connection between the reference electrolyte and the measured solution, can degrade the function of the reference electrode, since they bring about a redox potential in the sensor
Furthermore, a diffusion potential forms at the diaphragm between the reference electrolyte and the measured medium. The size and magnitude thereof depend, among other things, on the type and concentrations of the ions in the reference electrolyte and in the measured medium, on the type and geometric shape of the diaphragm and on the flow conditions. An attempt to minimize the diffusion potential or to hold it constant is made through the choice of a suitable reference electrolyte and a suitable embodiment of the diaphragm. Relatively low diffusion potentials can be achieved when a concentrated salt solution is used as a reference electrolyte, and moreover, when the cation and anion of the salt dissolved in the reference electrolyte have almost equal ionic mobilities. For this reason, a 3 to 4 molar aqueous solution of potassium chloride is frequently used as reference electrolyte or as bridge electrolyte in salt bridges. In potentiometric measuring, in general, the greatest part of the total measurement uncertainty rests on the uncertainty of the diffusion potential, even in the case of a carefully selected reference electrolyte.
The clogging by slightly soluble materials or other impurities previously described can significantly influence the diffusion potential in diaphragms made from porous materials, and therewith enlarge the measurement uncertainty or even corrupt the measured values to an unacceptable degree.
Numerous known approaches aim for reaching a high stability of the electrode voltage over time, i.e. a low sensor drift, and a long service life, by means of a special shaping of the diaphragm, in the case of which both the exit of the reference electrolyte into the measured medium and the entry of sample components in the reverse direction is small.
One the oldest known approaches, e.g. in K. Schwabe: pH-Messtechnik, Theodor Steinkopff, Dresden, 1976, is to connect the reference electrolyte and the measured solution via a plug shaped diaphragm made of a networked hydrogel. The gel plug suppresses convective mixing of the two solutions and at the same time represents a certain degree of diffusion barrier. In spite of this, the extraction of the electrolyte and the entry of disturbing components from the measured medium electrodes are still relatively high in the case of such reference.
Another possibility of an electrolytic connection between the housing interior of the reference electrode and the measured medium is in the embodying of the diaphragm as a gap, most often as an annular gap, or as a ground glass connection. Gap and ground diaphragms have a number of advantages, among these being that they are suitable for measurements in ion deficient media, the flow velocity of the measured solution scarcely influences the voltage; and the diffusion potentials and the electrical resistance are small. Additionally, ground diaphragms having a releasable ground piece are easily cleaned.
In the case of reference electrodes having a liquid reference electrolyte and a ground diaphragm, however, a sizeable loss of the electrolyte solution from the housing interior happens, so that the electrolyte must be replenished at times. Ground diaphragms are therefore suitable mainly for laboratory applications, however, less so for process measurements technology, in which maintenance free service life of the reference electrode is required for as long as possible.
If a gel electrolyte is provided in a reference electrode having a gap diaphragm, the loss of electrolyte from the housing interior is largely suppressed. There remains, however, a relatively strong diffusion of KCl from the reference electrolyte, from the housing interior into the measured medium, which leads to a potential drift of the reference electrode due to the concentration decline of the KCl. Moreover, components of the measured medium can diffuse into the electrolyte in the housing interior via the gap diaphragm.
A further approach for reducing mixing of reference electrolyte and measured medium is to make the diffusion path between the measured medium and the interior of the reference electrode as long as possible. Such a reference electrode is described in DE 102 07 624 A1, for example. In the case of spatially extended diffusion zones, however, an essentially constant diffusion potential, and, therewith, a stable, voltage measured value of the measuring chain, arises only gradually. Thus, in many cases the time response behavior of the potentiometric pH value measurement is determined not by the tuning processes of the pH selective, glass membrane of the measuring electrode, but, instead, by the tuning processes at the diaphragm of the reference electrode between reference electrolyte and measured medium.
A reference electrode having a single pore as a diaphragm, through which reference electrolyte escapes with a well defined and constant velocity, is described in CH 680 311 A5. In such case, length and diameter of the pore are so matched to one another that the electrical resistance of the electrolyte within the pore does not exceed a maximum range. For a pore diameter of 0.05 to 0.5 mm, the preferred length of the pore is between 0.5 and 12 mm, especially preferably between 7 and 8 mm.
Additionally, through the flowing out of the reference electrolyte with a constant velocity of 1 to 15 m per day, a constant diffusion potential and an equally constant response time should be assured. Through the significantly smaller internal surface area of the single pore compared to a porous material, sensitivity against fouling of the reference electrode by particles or disturbing substances from the measured medium should be reduced.
However, this embodiment has disadvantages: Through the flowing out of the reference electrolyte into the measured medium, the measured medium can be relatively strongly contaminated with the reference electrolyte. Furthermore, a pressure difference between the reference electrolyte in the interior of the reference electrode and the measured medium is required to assure flow of the reference electrolyte from the housing interior of the reference electrode into the measured medium. In the case of electrodes for use in the laboratory, such a pressure difference can be produced, in that the housing of the reference electrode has an opening in a region which is not immersed in the measured medium, through which a pressure equalization between the atmosphere and the housing interior of the reference electrode is achieved. The hydrostatic pressure of the reference electrolyte affected by a height difference of a few centimeters between the reference electrolyte in the housing interior of the reference electrode and the measured medium suffices for the reference electrolyte to flow out from the housing interior through the pore. In the case of applications in process measurements technology, in contrast, frequently an internal pressure production, for example, by means of a gas evolution cell in the housing interior; or an external pressure loading by means of pressurized gas; or electrolytes under pressure from an outer supply vessel, is required. However, these are relatively complex solutions, and therefore susceptible to defects and expensive. In addition to the apparatus effort, which must be pursued, in order to assure a continual flow of the reference electrolyte from the housing interior of the reference electrode, further effort is necessary, in order to limit the flow velocity to a maximum of 15 ml per day.